Recently a new class of semiconductor lasers, designated "quantum cascade" or "QC" lasers, was discovered. See, for instance, U.S. Pat. Nos. 5,457,709 and 5,509,025, U.S. patent application Ser. No. 08/825,286, titled "Article Comprising An Electric Field-Tunable Semiconductor Laser", filed Mar. 27, 1997 by Capasso et al., and U.S. patent application Ser. No 08/841,059, titled "Quantum Cascade Laser", filed Apr. 29, 1997 by Capasso et al., all incorporated herein by reference.
QC lasers are unipolar lasers that can be designed to emit radiation at substantially any desired wavelength in a rather wide spectral region. Specifically, they can be designed to emit in the mid-infrared (mid-IR), a spectral region in which there are few convenient radiation sources.
It is known that absorption lines of many target gases and environmental pollutants lie in the mid-IR spectral region, e.g., between about 3 and 13 .mu.m. This suggests that QC lasers could be advantageously used as radiation sources for absorption spectroscopy of such gases and pollutants, e.g., for emission monitoring or process control. Such use would be especially desirable because at least some QC lasers can operate in the relevant wavelength region at or near room temperature and with relatively high output power. See, for instance, J. Faist et al., Applied Physics Letters, Vol. 68, pp. 3680-3682 (1996); and C. Sirtori et al., IEEE Photonic Technology Letters, Vol. 9, pp. 294-296 (1997), both incorporated herein by reference.
However, we have come to realize that prior art QC lasers have a shortcoming that substantially decreases their usefulness as radiation sources for absorption spectroscopy and other applications. The shortcoming will be discussed below.
A "quantum cascade" or "QC" laser herein is a unipolar semiconductor laser having a multilayer structure that forms an optical waveguide, including a core region of relatively large effective refractive index between cladding regions of relatively small effective refractive index. A cladding region will also be referred to as a "confinement region". The core region comprises a multiplicity of nominally identical repeat units, with each repeat unit comprising an active region and a carrier injector region. The active region has a layer structure selected to provide an upper and a lower carrier energy state, and such that a carrier transition from the upper to the lower energy state results in emission of a photon of wavelength .lambda.. The carrier injector region has a layer structure selected to facilitate carrier transport from the lower energy state of the active region of a given repeat unit to the upper energy state of the active region of the adjacent (downstream) repeat unit. A carrier thus undergoes successive transitions from an upper to a lower energy state as the carrier moves through the layer structure, resulting in the creation of a multiplicity of photons of wavelength .lambda.. The photon energy (and thus .lambda.) depends on the structural and compositional details of the repeat units.